The present study was conducted to examine how daily exercise for several hours at a fixed time modifies the pattern of day-night variations in body core temperature and behavior in rats. Spontaneous wheel running was adopted as a model of exercise to avoid any artificial stress on rats.1. Male Wistar rats were acclimated to cages with a running wheels. Then, the running time of rats were limited to the first or last 3 or 6 h of the dark phase. After a 2-week activity restriction, the rats were again allowed access to the wheel freely. Wheel revolutions of rats during the period corresponding to the previous running time significantly increased after the activity restriction.2. Male Wistar rats were kept in cages with a running wheel and allowed access to the wheel for 6 h in the last half of the dark phase. After a 3-week exercise period, they were denied to run in the wheel. Their body core temperature significantly increased for 2-3 hours in the last half of the dark phase.The results suggest that, in rats, voluntary running limited to a fixed time daily alters the pattern of nycthemeral variations in body core temperature and locomotor activity, i.e., body core temperature and running activity increase during the period when the rats were previously allowed to exercise.かくのごとく、ラットの輪回し行動を一日の内一定時間帯に限定し、これに馴らすと、運動時間帯に対する記憶が形成され、かつての運動時間帯に一致して深部体温が上昇することが確認された。また、自発輪回し行動による運動鍛練によりラットの体温のセットポイントは上昇し、体内からの温熱負荷に対する体温調節反応は促進されることが示唆された。